1. Field of the Invention
The present invention relates to an oscilloscope sweeping circuit, and more particularly to an oscilloscope sweeping circuit which can precisely indicate a sweep rate.
2. Related Prior Art
A conventional sweep circuit of an oscilloscope is constructed, for example, as shown in FIG. 1. A current flowing through a resistor 1 with a constant voltage V1 supplied thereto charges a capacitor 2. A voltage across the capacitor 2 is amplified by an amplifier 3 which outputs a sweep signal. When the sweep signal reaches a predetermined level which is detected by a sweep end detector 4, a transistor 5 is driven to discharge the capacitor 2 to thereby terminate one sweep.
To vary the sweep rate of such a conventional sweep circuit, the supply voltage V1 or the resistance of the resistor 1 is changed.
In the latter method, various combinations of the capacitor 2 and resistance values of the resistor 1 are selected by means of a rotary switch 6 to cover a wide range of sweep rate, as schematically shown in FIG. 2A. Also, fine adjustment of the sweep rate is provided by connecting one end of the resistor 1 to a slide contact of a rheostat 7.
Particularly, as shown in FIG. 2B, there are provided a dial 8 for positioning the rotary switch 6 and a knob 9 coaxially mounted with the dial 8 for turning the slide contact of the rheostat 7. The dial 8 is rotated to stepwise change a sweep rate, whereas the knob 9 is rotated to achieve fine adjustment of a sweep rate. However, a known problem is that if the knob 9 is moved from a calibration position to another position to achieve fine adjustment of a sweep rate, the resultant sweep rate becomes uncalibrated. The reason is that a sweep rate set by rotating the dial 8 and indicated by a sweep rate gradation at mark A is in calibrated state only when a protrusion 10 fixed to the knob 9 is being set at calibration mark B.
Apart from the above, there is provided no means for obtaining sweep rate information from the above arrangement of varying a sweep speed using the rotary switch 6 and the rheostat 7. This becomes a problem especially for a read-out oscilloscope which always requires sweep rate information. Even if a rotational position of the knob 9 is detected using a rotary encoder, for example, the sweep rate cannot always be obtained with high precision due to the position of the slide contact of the rheostat at that time.
It is an object of the present invention to provide an oscilloscope sweeping circuit which can eliminate the above-described problems.